Friday, June 06, 2008
Crystallizing potassium dichromate
Difficulty: Hard
Solubility:15g/100ml of water
Time Required: hours for seed crystal, weeks for larger single crystal
---cooling crystallisation---
(seeding)
1.Heat some solvent to boiling. Place the solid to be recrystallized in a flask.
2.Pour a small amount of the hot solvent into the flask containing the solid
3.Swirl the flask to dissolve the solid.
4.Place the flask on the steam bath to keep the solution warm.
5.If the solid is still not dissolved, add a tiny amount more solvent and swirl again.
6.When the solid is all in solution, set it on the bench top. Do not disturb it!
7.After a while, crystals should appear in the flask.
8.filter the solution to isolate the crystals
9.Let the crystal finish drying on a watchglass.
10.Seeding Done.
(growing)
_in progress_
---evaporative crystallisation---
Another option is to obtain, at an approximately constant temperature, the precipitation of the crystals by increasing the solute concentration above the solubility threshold. To obtain this, the solute/solvent mass ratio is increased using the technique of evaporation. This process is of course insensitive to change in temperature (as long as hydration state remains unchanged).
All considerations on control of crystallization parameters are the same as for the cooling models.
---general---
what you need:
potassium dichromate
distilled water.
1.Dissolve as much potassium dichromate as you can in warm water.
2.Filter the solution, cover it, and allow it to sit undisturbed for several hours or until growth is observed. Alternatively, you could produce a seed crystal by evaporating a few drops of this solution in a shallow dish.
3.You can grow a mass of crystals by just allowing the solution to evaporate, but for a large single crystal, decant the solution into a clean container whenever you notice growth other than on your seed crystal(s).
4.You can control the growth of your crystal by changing the temperature of the solution or by controlling the rate of evaporation by the type of cover you put on the container (e.g., coffee filter has free airflow, sealing the container with plastic doesn't).
5.The resulting crystals will be bright orange rectangular prisms.
---enhancements---
Crystallization: Slow cooling vs rapid cooling
Slow crystallization gives larger crystals than this series of fast crystallization. Small crystals have a large surface area to volume ratio and impurities are located on the surface of the crystals as well as trapped inside the matrix.
Cotton or nylon?
Crystals don't adhere to nylon, so if you use it, you can get bigger, better crystals.
Potassium Dichromate
Stats:
Solubility in water 4.9 g/100 ml (0°C)
crystal structure:triclinic
Appearence:orange to red crystal
Stats:
Solubility in water 4.9 g/100 ml (0°C)
crystal structure:triclinic
Appearence:orange to red crystal
Density concentration table (+20°C)
